Polyvinylidene fluoride (PVDF) membranes are increasingly being used in microfiltration and ultrafiltration applications, and have been described in US patents such as U.S. Pat. Nos. 6,013,688 and 6,110,309. PVDF resin is very chemically and biologically inert and has outstanding mechanical properties. It is resistant to oxidizing environments, such as chlorine and ozone, which are widely used in the sterilization of water. The PVDF membranes are also highly resistant to attack by most mineral and organic acids, aliphatic and aromatic hydrocarbons, alcohols, and halogenated solvents. This resin is soluble in several solvents and may be readily solution cast to form porous membranes using a phase inversion method. PVDF membranes may be cast in either flat sheet or hollow fiber configurations.
Polyvinylidene fluoride microporous membranes, generally formed as thin sheets of substantially uniform thickness, have a sponge-like internal structure containing millions of intercommunicating channels, the channels having a substantially uniform width within narrow limits. The membrane pore sizes are generally controlled to be relatively uniform over a very small range. The pore size ranges generally fall within the larger general range of from 0.01 to about 10 microns.
PVDF membranes have been modified (post membrane formation) to improve specific properties. U.S. Pat. No. 6,734,386 describes post-treatment of PVDF membranes by polymerizing acrylic monomers on the surface of the membrane. Post-treatment reactions are complex and add costly steps to the manufacturing process.
PVDF is known to form miscible alloys with various acrylic resins. These include polymethylmethacrylate (PMMA), polymethylacrylate (PMA), polyethyl-methacrylate (PEMA), polyethylacrylate (PEA), and copolymers of these resins with other acrylic co-polymers. These miscible blends are stable over long periods. The acrylic polymers and co-polymers are not water soluble, and will not migrate with prolonged water exposure. (Polyvinylpyrrolidone (PVP) is another polymer resin that is miscible with PVDF, however, this resin is water-soluble and will migrate out of a blend on prolonged exposure to water. Therefore, PVDF-PVP blends will not be stable over long periods in water filtration use.) The acrylic resins have good chemical resistance of their own, and specifically will not undergo destructive dehydrofluorination that occurs with PVDF on exposure to caustic solutions.
U.S. Pat. No. 4,377,481 describes the use of PVDF-acrylic resin blends for polymeric membranes, in which the acrylic resin is a copolymer containing at least one monomer that if polymerized would be incompatible with PVDF—for example (meth)acrylic monomers containing sulfonic acid or amino groups.
U.S. Pat. No. 6,074,718 describes hollow fiber membranes of PVDF that may also be blends with a second polymer. The methyl methacrylate polymer described is a pore-forming material, which is extracted out from the final membrane. In this application it is not intended to remain in the final membrane, thus contributing no material benefit to the membrane.
One major limitation of PVDF membranes is caustic resistance due to attack by bases such as sodium hydroxide, potassium hydroxide, organic amines, and other bases. Sodium hydroxide solutions are often used for cleaning filtration membranes to remove biological fouling. This cleaning cycle reduces the lifetime of PVDF membranes and may lead to membrane failure. There is a need for a PVDF membrane with increased resistance to caustic cleaning to greatly improve the performance and lifetime of PVDF membrane modules. Filtration module lifetime is a major economic consideration for membrane filtration installations.
Surprisingly it has been found that membranes made of homogeneous PVDF-acrylic blends have improved caustic resistance.